This invention relates generally to motion compensation, and more particularly to improvements in heavy duty compensating devices making them simpler, more effective and reliable. More specifically, it concerns multiple actuators and control mechanisms therefor.
There is need on floating offshore drilling vessels for simple, effective, reliable, heavy duty, motion and load compensating equipment, to compensate for and nullify forces exerted as a result of deck "heave" on a vessel. A floating offshore drilling vessel cannot inherently provide a constantly stable platform as related to the sub-sea wellhead or bore hole. In this regard, a stable reference is required for landing and retrieving of wellhead and blowout prevention equipment, control of string weight on the drill bit in the hole, landing of casing and liner, coring, well logging, and tool fishing. There is need for nullification of the effects of rig/platform heave in response to swelling seas, and for compensating apparatus that will maintain a predetermined lifting force.
Prior Drill String Compensators (D.S.C.'s), sometimes called heave compensators, are of two types:
1. Block mounted, or PA1 2. Crown mounted
Block mounted compensators substantially increase the weight applied to the draw works, require precise alignment of derrick track and dollies, and represent a substantial change in the deck loading arm by their movement up and down the derrick.
Crown mounted compensators overcome these major disadvantages but still add a significant weight to the crown of the derrick. These two methods share some common disadvantages:
1. Stroke/compensation length is equal to rod length or must incorporate chains and sheaves which add additional wear/failure areas.
2. Rig heave compensation causes compression or expansion of compressed air, which in turn causes an inverse reaction in the compensating force applied.
There is also need for a heave tracking system to control drill string compensation.